Technical Field
The present invention relates to a handover technique, and particularly, to a technique for performing a handover by judging whether or not access by a terminal to a cell that is a handover destination is permitted or not.
Description of the Related Art
Techniques for performing a handover of a terminal engaged in communication between different cells have been conventionally known. For example, Section 10.1.2.1.1 of “3GPP TS 36.300 V8.3.0” prescribes a procedure of a handover between macro cells. FIG. 21 is a flow chart of a handover described in “3GPP TS 36.300 V8.3.0”. Hereinafter, a conventional handover procedure will be described with reference to FIG. 21.
First, a terminal (UE: User Equipment) receives a signal from a neighboring base station and measures propagation path quality. The terminal notifies a measurement result to a currently-connected base station (Source eNB, hereinafter referred to as “SeNB”) by means of a measurement report (“2. MEASUREMENT REPORTS” illustrated in FIG. 21). The measurement report includes a cell identifier (cell ID) of a base station having good propagation path quality as seen from the terminal, access information of the terminal (Tracking Area ID, hereinafter referred to as “TAID”), and the like. The SeNB determines a base station with a good propagation path status as a handover destination base station (Target eNB, hereinafter referred to as “TeNB”) of the terminal.
The SeNB has a list of neighbor cells (Neighbor Cell List, hereinafter referred to as “NCL”). An NCL is a list of cell IDs of base stations neighboring the SeNB, access information (TAD), and the like. The SeNB uses the NCL to transmit a handover request signal (“4. HANDOVER REQUEST” illustrated in FIG. 21) to a base station (TeNB) corresponding to a cell ID notified by the measurement report.
Upon receiving the handover request signal, the TeNB determines whether or not a handover can be performed based on the status of remaining resources and the like. When the TeNB determines that a handover can be performed, the TeNB transmits a handover response signal (“6. HANDOVER REQUEST ACKNOWLEDGE” illustrated in FIG. 21) to the SeNB.
Upon receiving the handover response signal, the SeNB transmits a handover execution instruction signal (“7. HANDOVER COMMAND” illustrated in FIG. 21) to the terminal. The handover execution instruction includes information necessary for the terminal for subsequent uplink synchronization such as a terminal identification ID (C-RNTI) in the TeNB.
Upon receiving the handover execution instruction, the terminal transmits a Random Access Preamble to the handover destination base station (TeNB) and starts the uplink synchronization (“9. SYNCHRONIZATION” illustrated in FIG. 21). Upon receiving the Random Access Preamble, the TeNB performs an uplink allocation for the terminal and notifies allocation information to the terminal (“10. UL ALLOCATION+TA FOR UE” illustrated in FIG. 21). When connection with the TeNB is successful, the terminal transmits a handover confirmation signal (“11. HANDOVER CONFIRM” illustrated in FIG. 21) to the TeNB and notifies that handover processing by the terminal has been completed. This concludes a basic outline of handover processing between macro cells.
The 3GPP LTE project is evaluating installing an indoor base station (Home eNB) in a home to construct a CSG (Closed Subscriber Group) cell. A plurality of CSG cells are provided in a single macro cell. Unlike a base station of a macro cell, a base station of a CSG cell is subjected to access restriction as seen in a table in Section 6.3.1a.3 in “3GPP TS 36.311 V8.0.0”. Therefore, a terminal is only able to connect to an access-permitted base station. Even if the terminal detects a base station with exceptional propagation path quality, the terminal is unable to connect to the base station without access permission.